Plastic magnets are usually produced by kneading magnetic powder having an average particle size in the range of 1-150 .mu.m and a thermosetting resin or a thermoplastic resin and then forming the resultant mixture by a conventional plastic molding method such as compression molding, injection molding or extrusion molding. Different from magnets obtained by sintering or casting, they have such features that they facilitate molding and machining and they enjoy elasticity and chemical resistance.
As magnetic powder for such plastic magnets, ferrite has conventionally been employed. Reflecting the recent trend toward still stronger magnets, it has been attempted to use as magnetic powder fine particles of intermetallic compounds which contain as principal components a rare-earth metal and an iron-group metal (hereinafter called "rare-earth magnets" for the sake of brevity) and which have higher crystallomagnetic anisotropy than ferrite, for example, a rare-earth magnet such as RCo.sub.5 - or R.sub.2 Co.sub.17 -base magnet, R being a rare-earth element, or an Nd-Fe-B-base magnet.
Upon production of a plastic magnet, such magnetic power is exposed to elevated temperatures during its kneading with a resin or during the molding of the resultant mixture. Since particles of a rare-earth magnet are, unlike ferrite, very susceptible to oxidation as mentioned for example in Japanese Patent Laid-open Nos. 16698/1979 and 71031/1979, they are oxidized in the course of their formation into plastic magnets. As a result, there is a problem that the magnetic characteristics of the resultant rare-earth plastic magnets are considerably inferior. In some extreme instances, their oxidation may proceed abruptly in the course of their formation into such plastic magnets, thereby raising another problem from the standpoint of safety.
Even when produced by a method featuring less chance of exposure to elevated temperatures during the production, for example, by a method such as compression molding, certain products may be used near the withstandable temperatures of their binder resins, leading to another problem that the magnetic characteristics of such magnetic powder may be deteriorated with time by their oxidation while they are used.
Many processes have already been proposed to produce rare-earth plastic magnets of high performance. As one of such proposals, it has been known to improve the performance of a magnet, which has been obtained by the powder method, by coating magnetic powder with a phosphorus compound so that the coefficient of the surface friction of the powder is lowered to increase the packing density of the resultant magnet powder in the molded magnet and the orientation thereof (see, for example, Japanese Patent Laid-open No. 26104/1982). This process characterizes the use of a phosphorus compound as a coating agent in lieu of an oil, paraffin, fluorinated resin which has conventionally been used.
A wide variety of phosphorus compounds may be used therein, including compounds between elements making up the magnetic powder and phosphorus, phosphorus-containing organic compounds and phosphorus-containing inorganic compounds. As specific examples, are mentioned manganese phosphate type, zinc phosphate type, iron phosphate type, zinc phosphate/manganese phosphate type, zinc phosphate/calcium phosphate type, etc. They are all well-known as principal components of phosphating or phosphate-pickling solutions for steel sheets. In other words, the above proposed processes feature application of the conventional phosphating or phosphate-pickling process, which have been used for ordinary steel sheets, to magnetic powder.
Magnetic powder produced by each of the above proposed processes is covered with the principal components of a phosphating or phosphate-picking solution as a lubricant thick layer on the surfaces thereof. Consequently, its bulk specific gravity was small and rare-earth plastic magnets obtained by using the powder as a raw material were not fully satisfactory in their performances (see, Comparative Example 4).